Since Xoops has a lot of code and I don't want to do this forever, everyone should document the code they write(...part of the xoops coding standards)! When you come across code that isn't documented yet, take a minute and write some doc for it. After changing any code, make sure that the documentation is still correct and up-to-date.
My work on the core classes is just to help get the documentation started. In the future everyone is responsible for documenting ALL CODE THEY WORK ON! Don't be put off by this. You will see it isn't much work and even helps you better understand what you are doing.